In general, HPA is synthesized by subjecting isobutylaldehyde and formaldehyde to an aldol condensation reaction in the presence of a basic catalyst.
The aldol condensation reaction may proceed either under acidic conditions or under basic conditions. However, since HPA contains a carbonyl group and a hydroxyl group in one molecule thereof, crystals of a dimer of HPA are condensed into a tetramer thereof under the acidic conditions. PTLs 1 and 2 disclose an aldol condensation reaction under basic conditions for the purpose of avoiding the condensation of the dimer of HPA into the tetramer.
After completion of the reaction, the resulting reaction product solution is subjected to distillation to remove low-boiling point components such as unreacted isobutylaldehyde and formaldehyde therefrom, thereby obtaining an aqueous solution containing HPA (hereinafter referred to merely as an “aqueous crude HPA solution”). HPA is frequently used as an intermediate product for synthesis of organic compounds such as neopentyl glycol and Spiro glycol, and as described in PTLs 3 and 4, the thus obtained reaction production solution may be used in subsequent steps without being subjected to any purification treatment.
Meanwhile, NPL 1 discloses that there is present an equilibrium relationship between a monomer of HPA and crystals of a dimer of HPA as shown in the following formula (1). Therefore, when HPA is subjected to crystallization purification, the HPA obtained as crystals are crystals of the dimer of HPA. The crystals of the dimer of HPA exhibit a reactivity substantially identical to that of the monomer of HPA as described in many literatures such as PTL 3, etc.

Also, there is disclosed a method in which the above aqueous crude HPA solution is diluted by adding water thereto, and then subjected to crystallization purification to obtain high-purity HPA (refer to PTLs 5 to 7). A filtrate obtained by subjecting a crystallization slurry to solid-liquid separation and by washing a filter cake contains uncrystallized HPA and/or a dimer of HPA. Such a filtrate may be reused as an intermediate product of organic compounds such as neopentyl glycol which can be produced without need of using high-purity HPA, or may be discarded.
The filtrate also contains a large amount of water. Therefore, in the case where the filtrate is reused as an intermediate product of the organic compounds, it may be sometimes required to previously remove water from the filtrate in view of deterioration in reaction efficiency, etc. Also, even when the filtrate is discarded, in view of a large load imposed on facilities for waste water treatments, it is preferred that the filtrate is separated into HPA and water before being discarded.
However, owing to such a fact that water and HPA have an azeotropic relation therebetween, there is a problem that it is difficult to remove only water from the filtrate by a distillation separation method to concentrate HPA in an efficient manner.
On the other hand, as the distillation separation technique, PTL 8 discloses a method of separating water from an aqueous hydroxypivalic acid solution by distillation using toluene or a mixture of toluene and 1-butanol, etc., as an azeotropic agent. PTL 9 discloses a method of producing Spiro glycol while subjecting HPA to azeotropic dehydration under reflux of toluene. However, PTLs 8 and 9 do not describe at all distillation separation between water and HPA and/or a dimer thereof. HPA has a chemical structure partially similar to those of hydroxypivalic acid and spiro glycol but contains different functional groups from those of hydroxypivalic acid and spiro glycol. Therefore, HPA has different properties from those of hydroxypivalic acid and spiro glycol. In general, in the case where water is separated from an aqueous solution by distillation using an azeotropic agent, the azeotropic agent capable of efficiently separating water from the aqueous solution may vary depending upon properties of substances dissolved in the aqueous solution. In consequence, the azeotropic agent to be used for the distillation separation must be carefully studied and selected in view of a system of the aqueous solution to be treated. Thus, an optimum azeotropic agent for a certain aqueous solution system is not necessarily an optimum azeotropic agent for another aqueous solution system, and therefore it is very difficult to predict an optimum azeotropic agent for a specific aqueous solution system.
PTL 10 discloses a method in which a mixed solution containing an aqueous sodium hydroxide solution, isobutylaldehyde, an aqueous formaldehyde solution and methanol is reacted to conduct an aldol condensation reaction between isobutylaldehyde and formaldehyde, and after completion of the aldol condensation reaction, methanol and unreacted isobutylaldehyde are removed from the obtained reaction product solution by azeotropic distillation. However, PTL 10 does not describe at all removal of water by the azeotropic distillation. In addition, in the aldol condensation reaction between isobutylaldehyde and formaldehyde as described in PTL 10, methanol that is unnecessary for the reaction is also added to the same reactor, so that a working efficiency of the reactor is considerably lowered, which results in industrially disadvantageous process.